Frequency‐adjustable magnetic field probes
Niklas Wehkamp, Philipp Rovedo, Elmar Fischer, Jürgen Hennig, Maxim Zaitsev
Abstract
Purpose
Nuclear Magnetic Resonance field probes provide exciting possibilities for enhancing MR image quality by allowing for calibration of k‐space trajectories and/or dynamic measurement of local field changes. The purpose of this study is to design and build field probes, which are easier to manufacture and more flexible to use than existing probes.
Methods
A new manufacturing method is presented based on light‐activated resin to encase the coil assembly and the 1H sample. This method allows for realizing field probes with tightly integrated orthogonal coils, whereby the local resonance frequency of protons can be adjusted during the MR experiment, by applying a DC current to the integrated 
‐field modification coil.
Results
The apparent field probe position in a gradient echo experiment was shifted within the field of view by changing its Larmor frequency using an integrated micro‐coil with 5.5 windings. The measured frequency modulation induced by the ‐field modification coil was 113 Hz/mA. The probe was tested with currents up to 100 mA. The DC current in the local field modification coil did not introduce visible artifacts in the MR images. Furthermore selective off‐resonant excitation of the new field probes at 2 kHz above the main RF frequency was demonstrated. Gradient impulse response functions measured with a traditional and proposed probe show similar gradient imperfections.
Conclusions
The presented approach opens up new possibilities for concurrent field monitoring during MR experiments using standard RF capabilities of clinical scanners.